1. Field of the Invention
The present invention relates generally to methods and systems for applying energy to tissue, and more particularly to methods and systems for injecting vapor media into an airway and causing a vapor-to-liquid phase state change to thereby apply thermal energy equivalent to the heat of vaporization of the vapor media into the lung. The delivery of energy is accomplished with a catheter in a minimally invasive procedure to shrink, seal and ablate a targeted region to reduce the effective volume of a patient's lung.
2. Description of the Related Art
Emphysema is a debilitating illness brought about by the destruction of lung tissue. The disorder affects up to 10% of the population over 50 years old. Emphysema is most commonly caused by cigarette smoking and, in some cases, by a genetic deficiency. The condition is characterized by abnormalities of the alveoli, which are the microscopic air sacs in the lung where gas exchange takes place. Destruction of these air sacs makes it difficult for the body to obtain oxygen and to get rid of carbon dioxide.
In emphysema, there is a progressive decline in respiratory function due to a loss of lung elastic recoil with a decrease of expiratory flow rates. The damage to the microscopic air sacs of the lung results in air-trapping and hyperinflation of the lungs. As the damaged air sacs enlarge, they push on the diaphragm making it more difficult to breathe. The enlarged air sacs also exert compressive forces on undamaged lung tissues, which further reduces gas exchange by the undamaged lung portions. These changes produce the major symptom emphysema patients suffer—dyspnea (shortness of breath) and difficulty of expiration. Current pharmacological treatments for emphysema include bronchodilators to improve airflow. Also, oxygen therapy is used for patients with chronic hypoxemia.
More recently, a surgical procedure called lung volume reduction (LVR) has been developed to alleviate symptoms of advanced chronic obstructive lung disease that results from emphysema. This surgical resection is variably referred to as lung reduction surgery or reduction pneumoplasty in which the most severely emphysematous lung tissue is resected.
The development of LVR was based on the observation that emphysema causes the diseased lung to expand and compress the normally functioning lung tissue. If the diseased lung tissue were removed, it was believed that the additional space in the chest cavity would allow the normal lung tissue to expand and carry on gas exchange. LVR was first introduced in the 1950's but was initially abandoned due to a high operative mortality, primarily due to air leakage. One of the main difficulties of the procedure is suturing the resected lung margin in an airtight manner. Normally there is a vacuum between the ribs and the lungs that helps to make the lungs expand and fill with air when the chest wall expands. If an air leak allows air in the potential space between the ribs and lungs—then the vacuum effect will disappear and the lungs will sag upon chest expansion making it increasingly difficult to inflate the lungs and perform gas exchange.
Currently, there are two principal surgical approaches for LVR—both of which involve removal of diseased lung tissue (typically in the upper lobes) followed by surgical stapling of the remaining lung to close up the incision. One approach is an open surgery in which the surgeon uses a median sternotomy to access the chest cavity for removal of diseased lung tissue. The second approach is a video-assisted thoracic surgery in which endoscopic instruments are inserted into the chest cavity through small incisions made on either side of the chest. LVR downsizes the lungs by resecting badly diseased emphysematous tissue that is functionally useless. Surgeons generally remove approximately 20-30% of each lung in a manner that takes advantage of the heterogeneity of emphysema in which the lesions are usually more severe at the apices and less severe at the lung bases. During the course of surgery, one lung is continually ventilated while the lumen of the contralateral lung is clamped. Subsequently, normal areas of the lung deflate as blood flows past the alveoli and resorbs oxygen, while emphysematous portions of the lung with less blood flow and reduced surface area remain inflated and are targeted for resection. The more recent procedures use bovine pericardium or other biocompatible films to buttress a staple line along the resected lung margin to minimize air leaks.
LVR improves function of the lung by restoring pulmonary elastic recoil and correcting over-distention of the thorax and depression of the diaphragm. Thus, the objective of LVR is to provide the patient with improved respiratory mechanics and relief from severe shortness of breath upon exertion. Many patients have reported benefits such as improved airflow, increased functional lung capacity and an improved quality of life. As in any major thoracic procedure, there are many risks, including fever, wound infections, wound hematomas, postoperative fatigue and tachycardia. The recuperation period following LVR varies from person to person, but most patients remain in the hospital for two weeks following surgery. The patient then must endure a regime of physical therapy and rehabilitation for several additional months. Further, the duration of the improvement in lung function following resection is not yet completely known—but there is a suggestion that lung function begins to decline two years after LVR. Despite optimistic reports, the morbidity, mortality and financial costs associated with LVR appear to be high, with some studies indicating mortality rates ranging from 4 to 17%.